Itec 275 computer networks switching routing and wans
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ITEC 275 Computer Networks – Switching, Routing, and WANs. Week 7 Robert D’Andrea. Some slides provide by Priscilla Oppenheimer and used with permission. Agenda. Learning Activities Domain Name Server (DNS) Summarization Root Owner DNS Routing tables Spanning Tree Protocol

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Itec 275 computer networks switching routing and wans

ITEC 275 Computer Networks – Switching, Routing, and WANs

Week 7

Robert D’Andrea

Some slides provide by Priscilla Oppenheimer and used with permission


  • Learning Activities

    • Domain Name Server (DNS)

    • Summarization

    • Root Owner DNS

    • Routing tables

    • Spanning Tree Protocol

    • Rapid Spanning Tree Protocol

    • Static versus Dynamic Routing

    • Routing Protocols and Characteristics

Interpreting a dns domain names
Interpreting a DNS domain names

DNS has a method of noting and interpreting the fully qualified path to a DNS domain name similar to the way full paths to files or directories are noted or displayed at a command prompt.

For example, a directory tree path helps point to the exact location of a file stored on your computer. For Windows computers, the back slash (\) indicates each new directory that leads to the exact location of a file. For DNS, the equivalent is a period (.) indicating each new domain level used in a name.

Interpreting file names
Interpreting File Names

UNIX uses the concept of relative and absolute file names. If a file name is preceded by a forward slash (e.g. /bin), the name is absolute. If the name is without a leading slash, it is considered relative to your current working directory.

Interpreting a dns domain names1
Interpreting a DNS domain names

For DNS, an example of a domain name with multiple levels is the following, a fully qualified domain name (FQDN):

Unlike the file name example, a DNS FQDN, when read from left to right, moves from its most specific information (the DNS name for a computer called "host-a") to its highest or most general piece of information (the trailing period (.) that indicates the root of the DNS name tree). This example shows the four separate DNS domain levels that lead away from the specific host location of "host-a":

Interpreting a dns domain names2
Interpreting a DNS domain names

1. The "example" domain, which corresponds to a subdomain where the computer name "host-a" is registered for use.

2. The "microsoft" domain, which corresponds to the parent domain that roots the "example" subdomain.

3. The "com" domain, which corresponds to the top-level domain designated for use by business or commercial organizations that roots the "microsoft" domain.

4. The trailing period (.), which is a standard separator character used to qualify the full DNS domain name to the root level of the DNS namespace tree.

Root servers
Root Servers

When a computer on the Internet needs to resolve a domain name, it uses resolver software to do the lookup. A resolver breaks the name up into its labels from right to left. The first component is queried using a root server to obtain the responsible authoritative server. Queries for each name are performed until a name server returns the answer of the original query.

Interpreting a dns domain names3
Interpreting a DNS domain names

As of 2013, there are 13 root name servers, with names in the form This does not mean that there are only 13 physical servers; each site uses redundant computer equipment to provide reliable service in when hardware and software fail on occasion.


Classful boundary summarization
Classful Boundary Summarization

Switching and routing choices
Switching and Routing Choices

  • Switching

    • Layer 2 transparent bridging (switching)

    • Multilayer switching

    • Spanning Tree Protocol enhancements

    • VLAN technologies

  • Routing

    • Static or dynamic

    • Distance-vector and link-state protocols

    • Interior and exterior

    • Etc.

Selection criteria for switching and routing protocols
Selection Criteria for Switching and Routing Protocols

  • Network traffic characteristics

  • Bandwidth, memory, and CPU usage

  • The number of peers supported

  • The capability to adapt to changes quickly

  • Support for authentication

Making decisions
Making Decisions

  • Goals must be established

  • Many options should be explored

  • The consequences of the decision should be investigated

  • Contingency plans should be made

  • A decision table can be used. Decision tables are composed of rows and columns. Each row corresponds to a single rule, with the columns defining the conditions and actions of the rules.

Transparent bridging switching tasks
Transparent Bridging (Switching) Tasks

  • Ethernet switches and bridges use transparent bridging.

  • A transparent bridge connects one or more LAN segments so that end systems on different segments can communicate with each other transparently. An end system sends a frame to a destination without knowing whether the destination is local or on the other side of the bridge.

Transparent bridging switching tasks1
Transparent Bridging (Switching) Tasks

  • Forward frames transparently

  • Learn which port to use for each MAC address

  • Flood frames when the destination unicast address hasn’t been learned yet

  • Filter frames from going out ports that don’t include the destination address

  • Flood broadcasts and multicasts


  • STP is a bridge protocol that uses the STA (Spanning Tree Algorithm) to find redundant links dynamically and create a spanning-tree topology database. Bridges exchange BPDU (Bridge Protocol Data Unit) messages with other bridges to detect loops.

  • BPDU STP hello packet that is sent out at configurable intervals to exchange information among bridges in the network.

Switching table on a bridge or switch
Switching Table on a Bridge or Switch

MAC Address








Cisco spanning tree protocol enhancements
Cisco Spanning Tree Protocol Enhancements

  • PortFastis a Cisco feature. It supports the concept of a switch edge port.

  • UplinkFast and Backbone Fast. UpLinkFast is a Cisco feature that is configured on access layer switches. Improves the convergence time of STP.

  • Unidirectional link detection is a hardware failure detection between switches.

  • Loop Guard is a Cisco product. Supports the prevention of loops caused by blocking port erroneously moving to the forwarding state.

Redundant uplinks
Redundant Uplinks




  • If a link fails, how long will STP take to recover?

  • Use UplinkFast to speed convergence

Distribution Layer

Switch B

Switch C


Primary Uplink

Secondary Uplink

Access Layer

X = blocked by STP

Switch A

Protocols for transporting vlan information
Protocols for Transporting VLAN Information

  • Inter-Switch Link (ISL)

    • Tagging protocol

    • Cisco proprietary

  • IEEE 802.1Q

    • Tagging protocol

    • IEEE standard

  • VLAN Trunk Protocol (VTP)

    • VLAN management protocol is a switch-to-switch and switch-to-router configuration.

Protocols for transporting vlan information1
Protocols for Transporting VLAN Information

  • VLAN Trunk Protocol (VTP)

    • The VLAN management protocol exchanges VLAN configuration changes as they are made to the network. VTP manages additions, deletions, and renaming of VLANs on a campus network without requiring manual intervention at each switch.

Selecting routing protocols
Selecting Routing Protocols

  • They all have the same general goal:

    • To share network reachability information among routers

  • They differ in many ways:

    • Interior versus exterior

    • Metrics supported hop count or bandwidth.

    • Dynamic versus static and default

    • Distance-vector versus link-sate

    • Classful versus classless

    • Scalability

Interior versus exterior routing protocols
Interior Versus Exterior Routing Protocols

  • Interior routing protocols are used within an autonomous system

  • Exterior routing protocols are used between autonomous systems

Autonomous system (two definitions that are often used):

“A set of routers that presents a common routing policy to the internetwork”

“A network or set of networks that are under the administrative control of a single entity”

Routing protocol metrics
Routing Protocol Metrics

  • Metric: the determining factor used by a routing algorithm to decide which route to a network is better than another

  • Examples of metrics:

    • Bandwidth - capacity

    • Delay - time

    • Load - amount of network traffic

    • Reliability - error rate

    • Hop count - number of routers that a packet must travel through before reaching the destination network

    • Cost - arbitrary value defined by the protocol or administrator

Routing algorithms
Routing Algorithms

  • Static routing

    • Calculated beforehand, offline

  • Default routing

    • “If I don’t recognize the destination, just send the packet to Router X”

  • Cisco’s On-Demand Routing

    • Routing for stub networks

    • Uses Cisco Discovery Protocol (CDP)

  • Dynamic routing protocol

    • Distance-vector algorithms

    • Link-state algorithms

Routing algorithms1
Routing Algorithms

Stub network has only one default path to non-local hosts and no outside network knowledge. Non-local stub network traffic uses a single logical path when traveling in and out of the network.

A good example would be an individual or group that uses only one router to link to an internet service provider (ISP). The individual or group are considered stub networks by the ISP.

Routing algorithms2
Routing Algorithms

The default route is the IP address of the next hop when no other routes are known.

To configure the default route to be

config t ip route

An interface can be used as an alternative to and IP address. To use serial0/0 for destinations not in the routing table, use:

ip route serial 0/0

Routing algorithms3
Routing Algorithms

A default route of a computer that is participating in computer networking is the packet forwarding rule (route) taking effect when no other route can be determined for a given Internet Protocol (IP) destination address. All packets for destinations not established in the routing table are sent via the default route. This route generally points to another router, which treats the packet the same way: If a route matches, the packet is forwarded accordingly, otherwise the packet is forwarded to the default route of that router. The process repeats until a packet is delivered to the destination. Each router traversal counts as one hop in the distance calculation for the transmission path.

Routing algorithms4
Routing Algorithms

Cisco’s On- Demand Routing

The Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol that, amongst other things, is used to discover other Cisco devices on either broadcast or non-broadcast media. CDP provides administrators with information that includes the IP address, software version, as well as the capabilities of the neighbor device. On-Demand Routing is an enhancement to Cisco Discovery Protocol that advertises the connected IP prefix or prefixes of a stub router via CDP. ODR also supports VLSM (Variable Length Subnet Mask), which means that it can be used in just about any network.

Routing algorithms5
Routing Algorithms

Cisco’s On- Demand Routing

It is important to know that ODR is not a routing protocol. Instead, it is simply an enhancement to CDP that is used to dynamically propagate routing information at Layer 2. The primary reasons ODR is often incorrectly referred to as a routing protocol is because it allows routers to dynamically exchange routing information. The second reason is because ODR is enabled using the router odrglobal configuration command.

Routing algorithms6
Routing Algorithms

Cisco’s On- Demand Routing

The primary benefits of using ODR is that it is not CPU intensive and it consumes very little bandwidth. 

Routing algorithms7
Routing Algorithms

Cisco’s On- Demand Routing

Static routing example
Static Routing Example

Router A

Router B

Router C








Host A

Host B

Host C

RouterA(config)#ip route

Send packets for subnet 50 to (Router B)

Default routing example
Default Routing Example

Router A

Router B

Router C








Host A

Host B

Host C

RouterA(config)#ip route

If it’s not local, send it to (Router B)

Distance vector routing
Distance-Vector Routing

  • Router maintains a routing table that lists known networks, direction (vector) to each network, and the distance to each network

  • Router periodically (every 30 seconds, for example) transmits the routing table via a broadcast packet that reaches all other routers on the local segments

  • Routers update their routing table, if necessary, based on received broadcasts

Distance vector routing tables
Distance-Vector Routing Tables

Router A

Router B

Router A’s Routing Table

Router B’s Routing Table

Network Distance Send To 0 Port 1 1 Router B

Network Distance Send To 0 Port 1 1 Router A

Link state routing
Link-State Routing

  • Routers send updates only when there’s a change

  • Router that detects change creates a link-state advertisement (LSA) and sends it to neighbors

  • Neighbors propagate the change to their neighbors

  • Routers update their topological database if necessary

Distance vector vs link state
Distance-Vector Vs. Link-State

  • Distance-vector algorithms keep a list of networks, with next hop and distance (metric) information

  • Link-state algorithms keep a database of routers and links between them

    • Link-state algorithms think of the internetwork as a graph instead of a list

    • When changes occur, link-state algorithms apply Dijkstra’s shortest-path algorithm to find the shortest path between any two nodes

Choosing between distance vector and link state
Choosing Between Distance-Vector and Link-State

Choose Distance-Vector

  • Simple, flat topology

  • Hub-and-spoke topology

  • Junior network administrators

  • Convergence time not a big concern

Choose Link-State

  • Hierarchical topology

  • More senior network administrators

  • Fast convergence is critical

Choosing between distance vector and link state protocols
Choosing between Distance Vector and Link State Protocols


  • Routing Information Protocol (RIP) Version 1 and 2

  • Interior Gateway Routing Protocol (IGRP)

  • Enhanced IGRP

  • Border Gateway Protocol (BGP)


  • Open Shortest Path First (OSPF)

  • Intermediate System-to-Intermediate System (IS-IS)

Routing protocol
Routing Protocol


Dynamic routing protocols
Dynamic Routing Protocols


Routing information protocol rip
Routing Information Protocol (RIP)

  • First standard routing protocol developed for TCP/IP environments

    • RIP Version 1 is documented in RFC 1058 (1988)

    • RIP Version 2 is documented in RFC 2453 (1998)

  • Easy to configure and troubleshoot

  • Broadcasts its routing table every 30 seconds; 25 routes per packet

  • Uses a single routing metric (hop count) to measure the distance to a destination network; max hop count is 15

Rip v2 features
RIP V2 Features

  • Includes the subnet mask with route updates

    • Supports prefix routing (classless routing, supernetting)

    • Supports variable-length subnet masking (VLSM)

  • Includes simple authentication to foil crackers sending routing updates

Igrp solved problems with rip
IGRP Solved Problems with RIP

  • 15-hop limitation in RIP

    • IGRP supports 255 hops

  • Reliance on just one metric (hop count)

    • IGRP uses bandwidth, delay, reliability, load

    • (By default just uses bandwidth and delay)

  • RIP's 30-second update timer

    • IGRP uses 90 seconds


  • Adjusts to changes in internetwork very quickly

  • Incremental updates contain only changes, not full routing table

  • Updates are delivered reliably

  • Router keeps track of neighbors’ routing tables and uses them as feasible successor

  • Same metric as IGRP, but more granularity (32 bits instead of 24 bits)

Open shortest path first ospf
Open Shortest Path First (OSPF)

  • Open standard, defined in RFC 2328

  • Adjusts to changes quickly

  • Supports very large internetworks

  • Does not use a lot of bandwidth

  • Authenticates protocol exchanges to meet security goals

  • Is an IP routing protocol that is completely link state.

Ospf metric
OSPF Metric

  • A single dimensionless value called cost. A network administrator assigns an OSPF cost to each router interface on the path to a network. The lower the cost, the more likely the interface is to be used to forward data traffic.

  • On a Cisco router, the cost of an interface defaults to 100,000,000 divided by the bandwidth for the interface. For example, a 100-Mbps Ethernet interface has a cost of 1.

Ospf areas connected via area border routers abrs
OSPF Areas Connected via Area Border Routers (ABRs)

Area 0 (Backbone)




Area 1

Area 2

Area 3

Is is

  • Intermediate System-to-Intermediate System.

  • Link-state routing protocol.

  • Designed by the ISO for the OSI protocols.

  • Integrated IS-IS handles IP also.

Is is1

IS-IS is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this by determining the best route for datagrams through a packet-switched network. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnections (OSI) reference design

Border gateway protocol bgp
Border Gateway Protocol (BGP)

  • Allows routers in different autonomous systems to exchange routing information

    • Exterior routing protocol

    • Used on the Internet among large ISPs and major companies

  • Supports route aggregation

  • Main metric is the length of the list of autonomous system numbers, but BGP also supports routing based on policies

Internet protocol egp
Internet Protocol (EGP)

During the early days of the Internet, EGP version 3 (EGP3) was used to interconnect Autonomous Systems. Currently, BGP version 4 is the accepted standard for Internet routing and has essentially replaced the more limited EGP3.

Internet control message protocol icmp
Internet Control Message Protocol (ICMP)

  • ICMP works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service for IP. Its messages are carried as IP datagrams.

Routing protocols summary
Routing Protocols Summary

Distant vector:







  • The selection of switching and routing protocols should be based on an analysis of

    • Goals

    • Scalability and performance characteristics of the protocols

  • Transparent bridging is used on modern switches

    • But other choices involve enhancements to STP and protocols for transporting VLAN information

  • There are many types of routing protocols and many choices within each type

Review questions
Review Questions

  • What are some options for enhancing the Spanning Tree Protocol?

  • What factors will help you decide whether distance-vector or link-state routing is best for your design customer?

  • What factors will help you select a specific routing protocol?

  • Why do static and default routing still play a role in many modern network designs?

This week s outcomes
This Week’s Outcomes

  • Spanning Tree Protocol

  • Rapid Spanning Tree Protocol

  • Static versus Dynamic Routing

  • Routing Protocols and Characteristics

Due this week
Due this week

  • 6-1 – Concept questions 5

Itec 275 computer networks switching routing and wans
Q & A

  • Questions, comments, concerns?